1. Field of the Invention
The present invention relates to an improvement in signal-to-noise ratio of regenerated signals in an optical disc apparatus.
2. Description of the Related Art
With regard to optical discs, the Blu-ray disc technology, which uses a blue semiconductor laser and a high NA objective lens, was commercialized, and optical systems have almost reached the limit of resolution of optical systems. An effective way of simultaneously implementing higher capacity and higher data transfer speed is multi-level recording. The techniques concerning the multi-level recording are described in Patent Documents 1 to 7 and the like.
Patent Documents 1 to 5 provide media in which the reflectivity of recording marks continuously changes with power of the recording light projected onto the recording media to implement recording at multiple levels. Patent Document 6 provides a method of modulating lengths and positions of recording marks in a predetermined cell to implement multi-level recording with patterns of the distribution of the amount of reflected light.
Patent Document 7 describes a technique to increase the recording capacity of a holographic memory for recording page data by recording the phase of information light at multiple levels. The outline of the technique will be described using FIG. 20. For recording data, first, a laser beam outputted from a light source 2001 is modulated by a spatial light modulator 2002 and then is made incident on a recording medium 2003 to record information. Herein, the spatial light modulator 2002 is composed of a number of pixels as shown in FIG. 21A. Light passing through central part of the spatial light modulator 2002 is subjected to phase modulation for each pixel to obtain information light (in the drawings, white part indicates a phase of 0, gray part performs phase modulation according to the color tone, and black part has a transmission of 0). Light passing through the periphery thereof is not modulated or is subjected to proper phase modulation to be reference light. An interference pattern of the information light and the reference light is recorded in the recording medium 2003. At the regeneration, at the spatial light modulator 2002, as shown in FIG. 21B, light passing through the central part of the spatial light modulator 2002 is not modulated and is DC light, and light passing through the periphery thereof is subjected to the same modulation as that for recording. The laser light having passed through the central part and the laser light having passed through the periphery are individually projected onto the recording medium. The projection of the reference light generates information light, and an image sensor 2004 detects light including the information light superimposed on the DC light. The detected light is interfering light of the information light and DC light. Each pixel of the information light is subjected to intensity modulation corresponding to the phase thereof for detection. Next, the phase of the DC light is equally changed by the spatial light modulator 2002, and similar measurement is carried out. The phase relationship between the information light and DC light at each pixel is then changed, and the image sensor 2004 therefore detects light having a different intensity pattern. The outputs of the image sensor 2004 are obtained in such a manner for four values of the phase of the DC light, 0, π/2, π, and 3π/2, thus providing the phase value of each pixel of the information light. In Patent Document 7, intensity modulation is also performed together with the phase modulation to further increase the number of levels.
Another approach to increase the recording capacity is being studied (see Non-patent Document 1 and Patent Document 8, for example). In this approach, two beams of light traveling in opposite directions are focused on a same place of a recording medium nearly to the diffraction limit like ordinary optical discs such as CDs and DVDs. The interference pattern (standing wave) of the two beams of light around the light focusing point is recorded. This approach can provide a surface recording density as high as conventional optical discs while multiple recording layers can be implemented easily, multiple recording is possible, and capacity can be increased easily. Moreover, the approach is not required to have tight tolerance as that in the page data hologram and can be implemented comparatively easily although the approach is a method of recording interference.    Patent Document 1: JP 2001-184649 A (EP 1235210 A)    Patent Document 2: JP 58-215735 A (GB 2122408 A)    Patent Document 3: JP 02-064932 A    Patent Document 4: JP 3559590 B    Patent Document 5: JP 61-211835A (U.S. Pat. No. 4,711,815)    Patent Document 6: JP 3033864 B (U.S. Pat. No. 5,555,231)    Patent Document 7: JP 2008-310924 A (US 2008/0310281 A)    Patent Document 8: JP 2007-220206 A (EP 1986187 A)    Non-patent Document 1: R. R. Mcleod et al., “Microholographic multiplayer optical disc data storage,” Appl, Opt., Vol. 44, 2005, pp. 3197